Artificial spinal disc

ABSTRACT

The present invention relates to methods and devices for the treatment of disc disease and spinal deformities with an artificial disc replacement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.11/534,014 filed Sep. 21, 2006 which is continuation-in-part of U.S.application Ser. No. 10/590,139 entitled “ARTIFICIAL SPINAL DISC”, anational stage filing of PCT/US2005/023134 filed Jun. 30, 2005, whichclaims the benefit of U.S. provisional applications 60/584,240 and60/658,161. The contents of all listed applications are incorporatedinto this application by reference.

TECHNICAL FIELD

The present invention relates to methods and devices for the treatmentof disc disease and spinal deformities with an artificial discreplacement.

BACKGROUND OF THE INVENTION

Spinal arthroplasty is an emerging field that offers the promise ofrestoring and/or maintaining normal spinal motion. The goal of spinalarthroplasty is to reduce or eliminate adjacent segment disease (ASD) bymaintaining the normal spinal biomechanics at the operative level. Toaccomplish this, an artificial cervical prosthesis must duplicate asclosely as possible the natural spinal biomechanics, includingmaintaining the axial height of the disc as well as applying angularadjustment throughout the full range of motion of the natural spine.

The spine plays an integral role in neural protection, load bearing andmotion. The vertebral column provides a strong, yet mobile central axisfor the skeleton and is composed of twenty-four vertebral bodies withseventy-five stable articulations. The intervertebral disc is afundamental component of the spinal motion segment, providing cushioningand flexibility. Adjacent vertebrae are linked together by threearticulations: a) the vertebral bodies and disc, which transmitcompressive and shear loads and provide flexibility, and b) by two facetjoints, which protect the disc from translational shear stress and limitrotation. This “triple joint complex” allows for flexion, extension,lateral bending and rotation of the spine.

The intervertebral disc is composed of an inner gel-like matrix calledthe nucleus pulposus and an outer surrounding fibrous band called theannulus fibrosus. When compressive loads are placed on the spine,increased pressure in the nucleus pulposus is transmitted to theannulus, which bulges outwards. The degenerative cascade of theintervertebral disc initially involves desiccation of the nucleuspulposus. With decreased elasticity and dampening from the nucleus,increased loads are transmitted to the annulus and facets. The increasedstress on the annulus can lead to fissures and radial tears in itscollagen fibers. With further degeneration, this can lead tocircumferential bulging of the disc, contained and uncontained discherniations, and complete desiccation of the disc. This degenerativecascade can result in axial pain, by stimulating pain fibers in theannulus, or compression of spinal nerves roots and/or the spinal cord.This can manifest itself in motor weakness, pain and/or numbness in thearms or legs or both.

The structure and function of the discs may be altered by a variety offactors including repeated stress, trauma, infection, neoplasm,deformity, segmental instability and inflammatory conditions.Degeneration of the intervertebral disc is the most common etiology ofclinical symptoms referable to the spine. Degeneration of the spine is auniversal concomitant of human aging. In the cervical spine, neck andarm pain caused by nerve root compression has been estimated to effect51% of the adult population. Spondylosis of the spine and aging areintimately related, with spondylosis increasing in both prevalence andseverity with age. Fortunately, the majority of patients will improvewithout surgery. In approximately 10-15% of cases, spondylosis isassociated with persistent nerve root and spinal cord compression and/orspinal pain, with a small percentage ultimately requiring surgery.

The most common type of surgery used in the United States for thetreatment of degenerative disorders of the spine (spondylosis) is spinalfusion. In an interbody fusion, the diseased disc is removed and eithera wedge of bone from the patient's hip, allograft or a metallic spaceris placed between the vertebrae where the disc was removed. Thisimmobilizes the functional spinal unit. While this surgery has beensuccessful in eliminating motion, there are disadvantages associatedwith it. By converting a mobile, functional spinal unit into a fixed,nonfunctional one, fusion results in increased strain patterns at levelsadjacent to the fused segment. When a segment of the spine is fused,there is elimination of motion at the level of surgery. Therefore, thestresses that would normally be absorbed by the disc at the site ofsurgery are now transferred to adjacent segments. This can causeadjacent segment disease (ASD) to one or several spinal units adjacentto the affected level. ASD can be defined as a clinical syndrome ofsymptomatic degenerative changes occurring adjacent to a previouslyfused motion segment. Retrospective studies have estimated that ASD canoccur in the cervical spine at a rate as high as 2.9% per year with aprojected survivorship rate of 26% at 10 years (Hilibrand A S, Carlson GD, Palumbo M, Jones P K, Bohlman H H: Radiculopathy and myelopathy atsegments adjacent to the site of a previous anterior cervicalarthrodesis. J Bone Joint Surg (Am) 81:519-528, 1999).

In the cervical spine, thousands of North Americans undergo surgery forcervical spondylosis each year. The majority of these procedures involvean anterior discectomy with decompression of the spinal cord and/ornerve root. The primary indication for surgery in the management ofcervical spondylosis is radiculopathy, myelopathy and/or neck pain.Following the discectomy, an anterior interbody fusion is commonlyperformed. Autologous bone harvested from the iliac crest or cadavericbone is most commonly used to fill the space created by the removal ofthe disc. A number of other solutions have been suggested, includingmetallic devices such as fusion cages or other types of spacers,xenografts such as bovine bone, and biological strategies such as theuse of growth factors. The graft for the interbody fusion can be shapedto correct underlying deformity of the cervical spine. By contouring thegraft one can restore lordosis to a straight or kyphotic spine.

A more recent alternative to spinal fusion is replacement of the damageddisc with a motion preservation device, which includes either a nucleusor total disc replacement (TDR). The rationale for the development ofthe artificial disc is to prevent adjacent segment disease. Artificialdisc devices can be broadly divided into two categories, those thatreplace the nucleus only, leaving the annulus and vertebral body endplates intact and those that involve replacement of the disc andaddition of prosthetic end plates. Both strategies are directed atrestoration of intervertebral disc function. Prosthetic nuclei aredescribed, for example, in U.S. Pat. Nos. 5,047,055 and 5,192,326.United States Patent application US2002/0183848 also discloses aprosthetic spinal disc nucleus that has a hydrogel core surrounded by aconstraining jacket.

There are several different types of prosthetic devices for use in thecervical or lumbar segments of the spine designed for TDR. For example,the Prodisc™ and the Charite™ disc are composites of cobalt chromium endplates with a polyethylene core. The Prodisc™ is described in U.S. Pat.No. 5,314,477 and the Charite™ disc is described in U.S. Pat. Nos.5,401,269 and 5,556,431. The Prestige™ disc is another type ofartificial disc that comprises a metal on metal design with a ball andtrough articulation. Another type of artificial disc that is gainingpopularity in the cervical spine is the Bryan® disc, described inseveral United States Patent applications including 2004/0098131;2004/00544411; and 2002/0128715. The Bryan® disc is a compositeartificial disc with a low friction, wear resistant, elastic nucleusthat articulates with two circular metal plates.

Presently, there are at least four artificial cervical disc replacementsystems undergoing clinical trials worldwide. These includeunconstrained devices, such as the PCM cervical disc. Theseunconstrained devices do not have mechanical stops to limit their rangeof motion. The Bryan® Cervical disc, the Prodisc™ C and the Prestige™ LPcervical disc systems limit range of motion to varying degrees. Thesesystems can be considered semi-constrained, in that there are mechanicalstops outside the normal range of motion. Thus far, only the Charite™disc has been approved for use in the United States.

Artificial spinal discs have been implanted for the management ofdegenerative disc disease producing radiculopathy, myelopathy and/oraxial spinal pain. More recently, artificial discs have been adopted forthe treatment of trauma. The aim of TDR is to reproduce the biomechanicsof the natural disc. Early clinical and biomechanical studies withsingle and multi-level disc replacement have reported favorable clinicaloutcomes and preserved range of motion at the level of surgery.Preservation of range of motion, however, while an important feature ofan artificial disc, is only a single measure of spinal biomechanics. Theeffect of the disc on angulation at the operative level, the averagedisc space height, and overall spinal alignment (sagittal and coronalbalance) also needs to be considered.

While the introduction of artificial discs has led to many successfulsurgeries, there are still problems associated with the current discs.For example, all of the current artificial cervical discs have a fixedheight across the entire disc. The artificial discs presently availablecan have issues with focal kyphosis or kyphosis at adjacent segments ofthe spine after the patient post-operatively reassumes an uprightposition, supporting the weight of the head and body. For instance, withthe Bryan® disc, the end plates are allowed to move freely about allaxes of rotation, allowing the end plate to assume a position resultingfrom the forces exerted on the implant by the head and neck. At times,this position may be significantly different from the positioning of thedisc intra-operatively. Several published studies with the Bryan®cervical disc replacement system have reported a tendency for the endplates of the prosthesis and the alignment of the cervical spine todevelop kyphosis following surgery. [Pickett G E, Mitsis D K, Sekhon L Het al. Effects of a cervical disc prosthesis on segmental and cervicalspine alignment. Neurosurg Focus 2004; 17(E5):30-35; Johnson J P,Lauryssen C, Cambron H O, et al. Sagittal alignment and the Bryan®cervical disc. Neurosurg Focus 2004; 17(E14):1-4; Sekhon L H S. Cervicalarthroplasty in the management of spondylotic myelopathy: 18 monthresults. Neurosurg Focus 2004; 17(E8):55-61.] This kyphotic angulationof the prosthesis has been attributed to the passive (unconstrainedmotion with a mobile nucleus and variable instantaneous axis ofrotation) design of the implant. None of the current TDR systemsaddresses this major complication.

A significant number of patients with spinal disc disease have a loss ofsagittal alignment of the spine as a result of the degenerative process.In addition, varying degrees of coronal imbalance can also occur. Noneof the available artificial disc replacement systems are designed torestore normal alignment to a spine that is straight, which havefocal/global kyphosis or coronal deformity. Existing artificial discreplacement systems that are inserted into either a straight, kyphoticor angulated segment are likely to take on the angle and localbiomechanics determined by the facets, ligaments and muscle forces. Assuch, patients with a pre-operative straight spine may developpost-operative kyphosis, and patients with a pre-operative kyphosis mayhave a worsening of the deformity post-operatively. Kyphosis of thespine has been implicated in segmental instability and the developmentof clinically significant degenerative disease. Several clinical studieshave described that a change in the sagittal or coronal balance of thespine can result in clinically significant axial spinal pain as well theinitiation and/or the acceleration of ASD. [Kawakami M, Tamaki T,Yoshida M, et al. Axial symptoms and cervical alignment after anteriorspinal fusion for patients with cervical myelopathy. J Spinal Disord1999; 12:50-60; Harrison D D, Harrison D E, Janik T J, et al. Modelingof the sagittal cervical spine as a method to discriminate hypolordosis:results of elliptical and circular modeling in 72 asymptomatic subjects,52 acute neck pain subjects, and 70 chronic neck pain subjects. Spine2004; 29:2485-2492; Katsuura A, Hukuda S, Saruhashi Y, et al. Kyphoticmalalignment after anterior cervical fusion is one of the factorspromoting the degenerative process in adjacent intervertebral levels.Eur Spine J 2001; 10:320-324; Ferch R D, Shad A, Cadoux-Hudson T A,Teddy P J. Anterior correction of cervical kyphotic deformity: effectson myelopathy, neck pain, and sagittal alignment. J Neurosurg 2004;100:S13-S19; Katsuura A, Hukuda S, Imanaka T, Miyamoto K, Kanemoto M.Anterior cervical plate used in degenerative disease can maintaincervical lordosis. J Spinal Disord 1996; 9:470-476.]

Attempting to provide a deformity correction by simply altering the endplate or the nucleus of an artificial disc, while still maintaining freemovement about all axes of rotation, may not be sustainable as theforces exerted by the head and body on the artificial disc couldcounteract the desired correction. To provide a sustainable correction,some limitation on the axes of rotation is required. From a designperspective, the goal is to design an artificial disc that is able tocorrect deformity (coronal and sagittal), has mechanical stops outsidethe normal range of motion (semi-constrained), and preferably hasvariable instantaneous axis of rotation (IAR).

The limits on the axes of rotation can fall into two categories. One isto provide correction using a permanent rotation or translation of anaxis to support the correction. This is accomplished using the geometryof the core and end plates themselves and is referred to the GeometricConstraint category. The second is to keep free range of motion aboutall axes but provide the correction using a material support. This typeof design provides the correction by the imposition of a deformablematerial in the plane of correction for normal rotation in that plane.This is the Material Constraint category of designs.

Degenerative disc disease is a major source of morbidity in our society.It can lead to serious economic and emotional problems for thoseafflicted. Thus, there is a need for an artificial disc that canalleviate both symptoms and correct deformity (sagittal or coronal orboth) of the spine.

BRIEF SUMMARY OF THE INVENTION

There are a number of different strategies that can be used with discreplacements to address the need for alignment/deformity correction inthe spine. With most of the available discs, the angle of disc insertioncan significantly alter the orientation of the prosthesis. This isrelated to bone removal and end-plate preparation for the prosthesis. Bychanging the angle of insertion, the disc can be placed either inparallel or at an angle to the disc space. Unfortunately, by changingonly the angle of insertion, one cannot correct an underlying deformityof the spine. Simply changing the angle of insertion is not adequate tocompensate for a device that does not have sufficient off-center loadbearing support or structure to maintain the correction of thedeformity.

A strategy to correct lordosis in the lumbar spine has been utilized bythe Link-Charite™ and Prodisc™ lumbar disc replacement systems by usingwedge-shaped end plates. A wedge-shaped end plate has also been used inat least one case with the Bryan® cervical disc system. However,wedge-shaped end plates are not routinely available at the present timefor cervical disc replacement systems. The strategy of usingwedge-shaped end plate(s) involves forming a differential thicknessacross the end plate. The articulation between the ball andsocket/trough or the nucleus and end plates is not altered, which is anadvantage because the complex geometry of how the prosthesis providesmotion is not altered. The disadvantage, however, is that this strategyis not forgiving if an error is made with either an overly corrected endplate or an end plate that is not corrected enough. The revision of theend plate can be difficult at the time of surgery and may even precludethe disc space from receiving a disc replacement. As most systems have acoating on the end plates that promote bony ingrowth, revision at alater date may be extremely difficult or even impossible. As there aretwo surfaces to the end plate, an outer surface that contacts the boneand an inner surface that articulates with the nucleus or core, it isconceivable that by changing the location or geometry of the innersurface, one could alter the center of rotation. This would be mostapplicable to prostheses that function as a “ball and socket”articulation. By changing the location of the “socket” or trough, thiscould alter how the prosthesis impacts alignment at the level of thedisc.

An alternate method of achieving lordotic correction is by changing thenucleus or inner core. The biggest advantage of this approach is thatthe nucleus or core can be more easily interchanged or revised.Intra-operatively, instruments can be used to gage the need for andamount of correction and the appropriate nucleus can be inserted. Bydesigning the correction into the nucleus, the surgeon is provided withflexibility and ease of insertion, and the ability for revision at alater date, which the other methods do not provide.

The invention includes a novel artificial disc that provides the normalrange of motion of the natural intervertebral disc, along with theability to correct deformity of the spine. The proposed disc allows forsemi-constrained range of motion of the functional spinal unit. It willreproduce the kinematics of the pre-operative normal spine. It willpossess maximum durability and biocompatibility, and a means forintegrating itself into the spine bony structure for long-termstability. Its insertion will be safe, simple, and ideally not addsignificantly to surgical time compared with the current procedures. Incontrast to the existing disc replacement systems, it will allow thesurgeon to correct deformity while maintaining natural kinematics of thespine.

A major advantage of this system will be that the nucleus may be easilyrevisable. For instance, in most cases where the Bryan® disc needsrevision, the entire disc, including the end plates, must be removed. Incases where the alignment of the spine changes with time, especially inchildren and young adults, this new disc replacement system will allowrevision of the nucleus, if needed.

The present invention addresses the problems associated with theartificial discs of the prior art by providing an artificial disc thatprovides for correction of spinal alignment deformity.

The artificial disc of the present invention is useful for the treatmentof degenerative disc disease including correcting spinal deformitiessuch as kyphosis, lordosis, and scoliosis.

It is an object of one aspect of the invention to provide an improvedartificial disc replacement that maintains motion at the operative leveland reduces the incidence of adjacent segment disease.

In one aspect of the invention, the artificial disc incorporates anartificial nucleus having an asymmetrical maximum vertical axis. Thepresent invention includes a non-spherical nucleus with a maximum pointof load-bearing and height in a non-central location (a differential inthe anterior/posterior height of the nucleus).

In one embodiment, the nucleus is adapted to provide lordodic correctionto a damaged spinal segment. In this case, the axis of greatest heightis positioned in the anterior part of the nucleus.

In another embodiment, the nucleus is adapted to provide kyphoticadjustment. In this case, the maximum height axis is positioned in theposterior part of the nucleus.

In yet another embodiment, the asymmetrical nucleus can be used for thetreatment of scoliosis. To achieve this, the axis of maximum height islateral (parasagittal) to the middle of the disc.

According to another aspect of the present invention, an artificialnucleus, or core, is provided for use in an artificial disc. The nucleuscomprises a body of biocompatible material, having the greatest verticalheight either at the central vertical axis or at a vertical axis otherthan the central vertical axis.

In another embodiment, the body is spherical or ovoid (egg-shaped),having convex upper and lower surfaces and a non-central maximum heightvertical axis. In an alternative embodiment, the nucleus is in the formof a truncated cylinder where the top is cut at a plane that is notparallel to the base. In another preferred embodiment, the disc isessentially circular.

It has been found that nucleus body designs with a completely roundedsurface (not necessarily spherical) have issues with reliablymaintaining correction when exposed to the variable forces of the headand neck. To address this issue, a segment or section that is flat orwhich has a contour different from the adjacent surface, can be formedin the central region of the nucleus body. This section will be referredto as a flattened section, which is meant to refer to any contour thatis not the same as the adjacent surface(s) of the nucleus. Such aflattened surface can be planar or it can have other shapes such as aslight convex or concave shape with a radius of curvature different fromthe adjacent surface. Such a flattened surface could also be in theshape of a compound curve or other complex shape. In the example ofproviding a lordotic correction, the flattened segment can be angledrelative to the superior end plate of the inferior vertebral body withthe height of the anterior part being greater than the height of theposterior part. The overall shape of the nucleus body is stillasymmetric, but the flattened segment is incorporated to provide areliable correction of the deformity. This flat segment providesstabilization of the correction by resisting misalignment moments actingthrough the nucleus. If the flattened segment is not of adequate size,there may be a tendency for the correction to disappear in the presenceof an anterior load or for a hyper-lordotic over correction in thepresence of a posterior load (during lordotic correction). An additionaladvantage of incorporating a flat segment in the nucleus is to providesurface contact over that area during small motions about the resting,neutral position of the device. This should help reduce wear on thedevice.

In another embodiment, the nucleus or core could be hemispherical inshape with a flattened inferior surface that fits in an opening ortrough formed in the lower end plate. Alternatively, the nucleus isasymmetric in that it has a greater vertical dimension or thickness onthe anterior aspect than on the posterior aspect in order to provide alordotic correction. The superior surface of the nucleus can have aflattened portion. The flattened portion may incorporate a concavesegment, but can have the other configurations as mentioned above. Theshape of the trough can be such that it defines the outer limits ofrotational or translational movement of the nucleus relative to thelower end plate. This design allows for greater ease of insertion of thenucleus without undue distraction of adjacent vertebrae because thetrough could be open at one end to allow for the nucleus to be inserted,and then a stop could be inserted in the trough to maintain the nucleusin the trough.

In another embodiment, instead of ovoid shaped nucleus, an elongated or“sausage type” shape can be used, which has spherical or ovoid endsections and a flattened or cylindrical center section. When a nucleusof this shape mates with a cylindrical bearing surface on the upper endplate, both surface and line contact are provided during lateral bendingas well as in flexion and extension. When this type of elongated nucleusis used, a corresponding end plate trough in the lower end plate can beprovided that allows for axial rotation with stops beyond the limits ofnormal motion. This trough can have the shape of a “bow tie,” “dog bone”or the like. The trough can be slightly oversized compared with thenucleus to allow limited anterior/posterior and medial/lateraltranslation. Additionally, the bearing surface of the end plate troughcan be curved upwardly at the outer limits of movement of the nucleus.This feature forces the nucleus to rise upwardly when it rotates andcause an axial distraction of the device that forces the adjacentvertebral bodies apart and loads the tissues between them, resulting ina gradual stop to the motion. The translation of the core within thetrough attempts to preserve the mobile instantaneous axis of rotation ofthe natural disc.

In another embodiment, an elongated or “sausage type” shape nucleus isshaped so that the superior surface of the nucleus possesses adepression or valley formed in the flattened section, which extendsalong the sagittal plane. This can be accomplished, for example, byremoving material from the central region of the flattened segment ofthe nucleus, creating a valley between the side portions. The sideportions are contiguous with the remaining elements of the nucleus, anddo not protrude in the vertical plane. The side portions are preferablysymmetrical about the sagittal plane.

Additionally, the trough can be open at the anterior end to allow forinsertion of the nucleus without excessive distraction of the adjacentend plates. A locking mechanism can be provided to prevent the nucleusfrom being expelled from the trough after insertion of the nucleus.

In another aspect of the invention, a novel type of end plate isprovided. Unlike other end plates, which require extensive preparationof the vertebral body surface, the present end plates have anessentially flat outer or vertebral-contacting surface that allows themto be easily inserted. In a preferred embodiment, the surface is asemi-round plate having at least one unidirectional keel for anchoringthe plate in position. The outer surface of the end plate may be treatedin a way that promotes bony ingrowth to enhance stability of the endplate in situ. In one embodiment, the outer (vertebral-contacting)surface and the inner (nucleus-contacting) surface are essentiallyparallel to each other. In another embodiment, the outer surface and theinner surface are non-parallel thereby giving the end plate anessentially wedge-like configuration. The orientation of the wide andnarrow edges of the wedge can be adjusted to provide various types anddegrees of spinal correction.

In another aspect of the invention the prosthesis comprises anartificial nucleus and at least one end plate. In this embodiment, theprosthesis comprises a superior end plate for attachment to an uppervertebral member, an inferior end plate for attachment to a lowervertebral member and a nucleus adapted to fit between the two endplates. The end plate of the invention has a generally flat surface onthe bone contacting side and the appropriate geometric receptacle on theother side for articulating with the nucleus. A central keel can beformed in the center of the inner surface of the end plate to anchor thenucleus in position. The end plate can include a stop member to preventthe prosthesis from moving toward the spinal canal. The nucleus may alsohave a maximum vertical axis that is not at the geometric center.

In another embodiment, the nucleus has an upper surface with an upperreceptacle and a lower surface with a lower receptacle. The superior endplate has a downwardly projecting protrusion or anchor that engages theupper receptacle and the inferior end plate has an upwardly extendingprotrusion or anchor that engages the lower receptacle. The prosthesismaintains an appropriate spatial relationship between adjoiningvertebrae and also permits normal range of motion of the spine. Thisembodiment can also include a receptacle that comprises a groove open atone end. The anchor on the end plate can include a central keel, whichslides into position in the groove to secure the nucleus.

Another embodiment of the invention operates like a universal joint andincorporates three anatomical axes of rotation, two of which provide forflexion/extension and lateral bending motion, while the other oneprovides for axial rotation. These axes of rotation are accomplished bythe use of a pair of two cylinders that can rotate relative to eachabout a central post.

In another embodiment, one of the plates has a central post that engagesthe other plate, and an annular core positioned around the central postthat is formed of a resilient material. The core can be asymmetrical andengage both plates to provide necessary deformity correction. The corecan engage the end plates to provide the desired angle between theplates for deformity correction, with the central post engaging theother plate when the load exceeds a predetermined limit. Or, the postcan engage the other plate with the core engaging the other plate tomaintain the plates at the desired angle relative to each other whenapplied forces tend to change the relative angle of the plates.Alternatively, the core could be replaced by two or more discretespacers for performing the same function.

In another aspect of the invention, the nucleus can utilize materialdeformation to accomplish the desired ranges of motion. The shape of thematerial can be used to provide a restoring force for deformitycorrection. In order to achieve these results, material can be removedfrom various parts of the core to change the modulus of elasticity ofthe core at selected locations, or material having variable elasticmodulii could be used. In this way, different forces and motions can beprovided though the design of the core.

The end plates can be provided with features that act as stops outsideof the desired range of motion, which allow for anatomically-derivedgradual stopping. This result can be achieved by forming one or morecamming surfaces in or on one of the end plates and providing aco-operating member on the other end plate for engaging the cammingsurface. The camming surface has a gradual curve on its inner surface.During relative movement between the end plates, the camming surface isengaged by the cooperating member, which results in an axial distractionof the end plates and provides a soft tissue assist to prevent a hardstop. Alternatively for rotational movement, cooperating cammingsurfaces can be provided so that distraction will occur when one endplate rotates relative to the other one.

In another embodiment, the nucleus has a tang or tab protruding in theposterior direction from the inferior aspect of the body of the nucleus(core). The tab interacts with the inferior endplate to resist “liftoff” of the nucleus from the inferior endplate, thus preventingposterior migration (expulsion) of the nucleus into the spinal canal. Ina more preferred embodiment, the underside of the tab is chamfered orbeveled.

In another embodiment, the posterior superior surface of the nucleus iscurved upward from the medial superior surface to provide an elevatedposterior surface region relative to the center of the nucleus. In thefull extension position of the prosthesis this configuration mayreposition the instantaneous axis of rotation to a more superiorlocation and allows the endplates to resist posterior shear. Posteriorshear load is transmitted through the nucleus and into the inferior endplate rather than through the facet joints and related soft tissuestructures.

In some embodiments, recesses on the anterior portion of the nucleusprovide access for external instrumentation to facilitate placement andremoval. These recesses are placed in an area such that they do notsubstantially interfere with the load carrying and transferringcapabilities of the nucleus.

In some embodiments, a polymer is incorporated on one or more of thearticulating surfaces. In one preferred embodiment, a component with anarticulating surface is molded from the polymer. In a second preferredembodiment the polymer is incorporated by insert-molding as a part ofthe component. A preferred polymer for these embodiments ispolyetheretherketone (PEEK). In another embodiment, ceramics oralternate materials such as zirconium oxide can be utilized.

The invention also includes a method for implanting spinal prostheses ofthe type described above, and instruments for performing such a methodof implantation. The method includes the steps of distracting a pair ofadjacent vertebral bodies to a specific disc space height, maintainingthe height between vertebral bodies with a first instrument that canoperate to guide subsequent instruments for forming vertebral grooves onthe adjacent vertebral bodies, forming vertebral grooves on the facingsurfaces of the vertebral bodies that correspond with keels on the outersurfaces of the prosthesis by using the second instrument to guide drillbits; and inserting the prosthesis with the nucleus sandwiched betweenthe end plates between the vertebral bodies with the keels beinginserted into the vertebral grooves. The method also includes the stepsof forming starter grooves with the second instrument and shaping thestarter grooves into grooves with a third instrument, forming thegrooves with a single instrument, and determining the size, shape anddegree of lordosis to be accommodated before performing the step offorming grooves.

The set of instruments includes a first instrument with a pair ofprojections adapted to be inserted between a pair of adjacent vertebralbodies for maintaining the vertebral disc height, and a guide surfacefor guiding one or more other instruments into a predetermined positionbetween the adjacent vertebral bodies. A second instrument includes aprofile for engaging the guide surface of the first instrument forinsertion between the vertebral bodies into a predetermined position,and a plurality of guide surfaces for guiding drill bits for forminggrooves in the vertebral bodies that correspond with the keels formed onthe outer surfaces of the end plates. The instruments can also include aset of trial instruments with a profile for engaging the guide surfaceof the first instrument for insertion between the vertebral bodies intoa predetermined position, the trial instruments being sized and shapedfor determining the size of the implant and the degree of lordosis to beaccommodated. The instruments can include a set of trial instrumentsthat are gauged to measure at least 0°, 3°, 6° and other varying degreesof lordosis.

A third instrument can be included that includes a profile for engagingthe guide surface of the first instrument for insertion between thevertebral bodies into a predetermined position, and a plurality ofcutting surfaces for shaping the grooves to correspond with the shapesof the keels when the third instrument is moved back-and-forth relativeto the first instrument. The guide surfaces in the third instrument canbe oblong for allowing the drill bit to move superior-inferior,medial-lateral relative to the axis of the grooves. A plurality of guidesurfaces on the second instrument can be used for guiding drill bits forforming grooves in the vertebral bodies that correspond with the keelsformed on the outer surfaces of the end plates. The guide surfaces canbe shaped to form an unequal number of grooves in the adjacent vertebralbodies.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set fourth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1A illustrates an spherical artificial disc nucleus with themaximum central axis in the geometric midline of the nucleus;

FIG. 1B illustrates the nucleus of FIG. 1A, with an offset maximumvertical axis that provides 3° of correction;

FIG. 1C illustrates the nucleus of FIG. 1A, with an offset maximumvertical axis that provides 6° of correction;

FIG. 2A illustrates an asymmetrical artificial disc nucleus with themaximum central axis in the geometric midline of the nucleus;

FIG. 2B illustrates the nucleus of FIG. 2A with an offset maximumvertical axis that provides 3° of correction;

FIG. 2C illustrates the nucleus of FIG. 2A with an offset maximumvertical axis that provides 6° of correction;

FIG. 3 is a top view of the embodiment of the artificial disc nucleusshown in FIG. 1A;

FIG. 4 is a perspective view of the embodiment of the artificial nucleusshown in FIG. 1A;

FIG. 5 is a perspective view of the embodiment of the artificial nucleusshown in FIG. 2A;

FIG. 6 is a perspective view of an outer surface of an end plate;

FIG. 7 is a perspective view of an inner surface of an end plate;

FIG. 8 is a front view of an end plate;

FIG. 9 is a front view of a spinal disc device with the nucleus shown inFIG. 1A;

FIG. 9A is a front view of a pair of endplates with an offset keelconfiguration;

FIG. 10 is a side view of the spinal disc device of FIG. 8;

FIG. 11 is a front view of a spinal disc device with the nucleus shownin FIG. 2A;

FIG. 12 is a side view of the spinal disc device of FIG. 8;

FIGS. 13A and 13B illustrate an embodiment of an artificial spinal discprosthesis where the end plates may be adapted for lordotic correction;

FIGS. 14A, 14B, and 14C illustrate other embodiments where the endplates can be adapted for lordotic correction;

FIG. 15 is a side view of another embodiment which provides for alldirections of movement;

FIGS. 16A and 16B illustrate the two sections of the nucleus of theembodiment of FIG. 15;

FIGS. 17 and 18 illustrate another embodiment of the invention in whichthe nucleus is formed of upper and lower sections with an intermediatesection;

FIG. 19 illustrates another embodiment of the invention in which thenucleus is cut in half and has a flat lower inferior surface;

FIG. 20 is a schematic view of the nucleus of FIG. 19;

FIG. 21 illustrates a modification of the embodiment of FIG. 19;

FIG. 21A illustrates a nucleus with an asymmetric thickness and aconcave superior surface that is designed to also provide a lordoticcorrection.

FIG. 22 is a an underside view of the nucleus of FIG. 21;

FIG. 23 is a schematic view of the nucleus of FIG. 21;

FIG. 24 illustrates a modification of embodiment of FIG. 19;

FIGS. 25-31 illustrate another embodiment of the invention in which thenucleus is elongated with a flattened section in the center;

FIGS. 32 and 33 illustrate another embodiment of the invention whichutilizes a universal joint;

FIGS. 34-36 illustrate another embodiment of the invention in which aresilient ring and a post provide for relative motion between the endplates;

FIG. 37 illustrates a modification of the embodiment of FIG. 34;

FIGS. 38 and 39 illustrate another embodiment of the invention in whichthe nucleus is shaped to provide medial/lateral correction; and

FIGS. 40-43 illustrate another embodiment of the invention in which theend plates are provided with stops outside the normal range of motion.

FIGS. 44A, 44B and 44C illustrates another embodiment of the inventionin which the flattened segment of the nucleus contains a centraldepression;

FIGS. 45, 45A, 45B and 45C illustrate another embodiment of theinvention in which the posterior superior surface of the nucleus has anelevated surface region;

FIGS. 46A, 46B, 46C and 46D illustrate another embodiment of theinvention in which a modified keel configuration is shown;

FIGS. 47-53 are various views of instruments for implanting spinalprostheses of the type described above, and for illustrating a preferredmethod of implantation; and

FIGS. 54, 55 and 56 illustrate another embodiment of the invention whichis a two piece artificial spinal disc prosthesis.

DETAILED DESCRIPTION OF THE INVENTION

In its proper, healthy alignment, the spine follows natural curves,which promote proper sagittal and coronal balance (flexibility) andallow for balanced load sharing between the vertebrae. These curvesinclude the cervical, thoracic, lumbar and sacral regions of the spine.Naturally, in order to accommodate a curve, there must be some variationin the angle of articulation between the functional spinal units and theheight of an intradiscal space. The cervical and lumbar regions arenaturally lordotic, or curved convexly in the anterior direction. Atdifferent segments along the spine, there are typically differentheights for the vertebral bodies and the intradiscal space. In addition,the intradiscal space and vertebral body height may be different fordifferent people.

Each intradiscal space has anterior and posterior regions. An artificialdisc in the cervical, thoracic and lumbar regions that maintain the sameheight from the anterior to the posterior may promote an abnormalalignment, resulting in additional stress at the anterior or posteriorportions of an adjacent disc. It may also result in an uneven loaddistribution across the device and cause an excessive amount of relativemotion, wear debris and early failure,

As used herein, the terms, nucleus and core are used interchangeably torefer to an artificial intervertebral device that replaces a damagednatural spinal disc. The artificial core may be provided alone or incombination with a superior end plate for attachment to an uppervertebra or an inferior end plate for attachment to a lower vertebra orboth.

The terms “upper” and “lower” are used herein to refer to the vertebraeon either side of the disc to be replaced, or a surface on a part in theposition shown in the referenced drawing. A “superior” plate is affixedto an upper vertebra and an “inferior” plate is affixed to a lowervertebra of a functional spinal unit.

The terms vertical and horizontal are used herein relative to a standinghuman being in the anatomical position. The term “anterior” refers tothe region towards the front and the term “posterior” refers to theregion towards the back. The term “sagittal” refers to regions on eitherside of the central midline axis of a standing human being.

The term “asymmetrical” is used herein to refer to an axis of maximumheight that is not placed centrally or to a nucleus or total discreplacement (TDR) not having its maximum vertical axis placed centrally.In other words, the maximum height is not situated or pivoted at acenter line of symmetry so that the TDR comprises regions that are notexactly the same in shape or size as other regions on the other side ofa line of symmetry. The location of maximal load bearing is located in anon-central location.

In one embodiment of the present invention, an artificial disc comprisesa nucleus that is not geometrically symmetrical. The disc may have amaximum vertical axis that is not located at the geometric center of thedisc. The maximum vertical axis may be located toward the front of thedisc, the rear of the disc or on one side of the disc. The positioningof the maximum vertical height and load bearing capability is chosendepending on the type of deformity that needs to be corrected. Thepresent invention also provides methods for the treatment ofdisc/vertebral body disease, lordosis, kyphosis and scoliosis using anasymmetric artificial disc.

One advantage of the present invention is that the “nucleus” or core maybe interchanged and revised intra-operatively and post-operatively.Instruments can be used to gauge the need for and amount of correctionand the appropriate implant can then be inserted. By introducingcorrection into the nucleus, the surgeon benefits from flexibility, easeof insertion and revisability that present systems do not provide.

Artificial discs of the present invention can be provided with variousdegrees of deformity correction. For this aspect of the invention, thesurgeon can choose a disc having the appropriate correction for thepatient. Thus, a method of treating a spinal deformity is provided. Thismethod comprises preparing a spinal segment for implantation of anartificial disc, determining the desired angle of the intervertebralspace, selecting an artificial nucleus having the desired dimensions,affixing a superior end plate to the upper vertebra, affixing aninferior end plate to the lower vertebra and inserting the selectednucleus between the superior and inferior end plates. Alternatively, andthe assembled unit of end plate-nucleus-end plate may be inserted inunison. The configuration of the nucleus in this pre-assembled constructcan be determined by the intra-operative measurement tools, or withpre-operative calculations. Pre-operative planning techniques andinstruments may also be able to determine the size and orientation ofthis device for insertion.

A major advantage of the present system is that the artificial disc canbe more easily and rapidly inserted and the nucleus can be changed orrevised in accordance with the magnitude of the deformity beingcorrected. This is especially useful in children and young adults wherethe alignment of the spine changes over time.

In one embodiment, an asymmetric nucleus adapted for lordotic correctionof the cervical spine is provided. The surgeon can restore lordosis tothe cervical spine while maintaining motion. The nucleus may be composedof a low friction elastomer such as polyurethane,polycarbonate-polyurethane, a polymer such as polyethylene (particularlyultra-high molecular weight polyethylene), a suitable ceramic, metals ormetal alloys such as titanium or a titanium alloy,chrome-cobalt-molybdenum (CoCrMo), cobalt 28 chromium molybdenum, cobaltchrome, stainless steel, or other suitable materials. It has a generallycircular geometric design, with varying degrees of lordosis incorporatedinto it by utilizing an axis of maximum height anterior to the geometriccenter of the nucleus. The anterior height of the nucleus varies,depending on the extent of lordotic correction needed. The nucleus isavailable in various lordotic angles, e.g. 0, 3° and 6°, as well asdiffering heights (e.g., 4, 6 and 8 mm). Before deciding on the finalnucleus size, a set of instruments or other means can be used to gaugethe need for lordotic correction.

The nucleus slides between a superior end plate and an inferior endplate. The nucleus can be maintained in position using various types ofconnectors. For example, in one embodiment, the convex surface of thenucleus has a midline groove to allow the nucleus to slide into placebetween the positioned end plates. A central keel on the concave surfaceof the end plate is received in the groove of the nucleus. It isapparent that other types of connections can be used to maintain thenucleus in position. For example, a tooth and lock system or a pop-insystem could be used.

A number of embodiments of the nucleus and artificial disc of thepresent invention are illustrated in the appended drawings. In oneaspect of the invention, correction of spinal segment alignment isprovided by an artificial nucleus which has the shape of a truncatedcylinder or which is generally spherical or ovoid in shape, wherein thetwo halves on the arc on either side of a central axis are notsymmetrical. In other words, the curvature is not geometrically parallelor symmetric.

In one embodiment, the implant consists of three pieces. The end plateswill be made in differing sizes to accommodate differences in anatomy.These may be fabricated of titanium or a titanium alloy,chrome-cobalt-molybdenum (CoCrMo), cobalt 28 chromium molybdenum, cobaltchrome, stainless steel or other materials suitable for spinalprosthetic inserts.

The end plates can have two distinct surfaces. The flat surface of eachend plate, which contacts the vertebral body end plate, is capable ofaccommodating bony ingrowth and incorporates a suitable coating, such asporous titanium, a calcium phosphate, or includes other types of knownsurfaces that promote bony ingrowth for long-term stability. The endplates can also have one or more parasagittal keels that provideimmediate fixation. In one embodiment of the invention, a pair ofparallel keels can be formed on the outer surface of one of the endplates, and a single, centrally-located keel can be formed on the outersurface of the other end plate. The other (inner) surface of the endplates can have a contour that corresponds with the geometric shape ofthe nucleus to form a bearing surface that allows for optimalarticulation and wear characteristics with respect to the nucleus. Inthe middle of this bearing surface, there can be a single, central keel,which provides a constraint for the nucleus against excessivetranslation and range of motion. The nucleus can have a circulargeometric design, with a midline groove to allow the nucleus to slideinto place between the positioned end plates. A central keel on theconcave surface of the end plate would fit into the groove of thenucleus. Before deciding on the final nucleus size, a set of instrumentscould be inserted to confirm the lordotic correction, but these may alsobe used as confirmation for other types of presurgical planningtechniques and instrumentation. Alternatively, intra-operativeinstruments may be used as confirmation for other types of pre-surgicalplanning techniques and instrumentation.

FIGS. 1A to 1C illustrate various examples of artificial disc nucleiwhere the nucleus is symmetrical, with a maximum central axis in thegeometric center 20 of a nucleus 10. The reference letters A and Pillustrate the anterior and posterior orientation, respectively, of thenuclei 10, 14 and 18. The nucleus 10 is generally spherical in shape andis truncated with a flattened portion 22A on the upper side of thenucleus 10 and another flattened surface 22B on the lower side. Thenucleus also has upper and lower curved surfaces 24A and 24B,respectively, and a circumferential wall 26.

The flattened surfaces, as described above, can be advantageous becausewhen the nucleus has a completely rounded surface, it cannot reliablymaintain correction when exposed to the variable forces of the head andneck. A flattened surface incorporated into the central region of thenucleus can be used to solve this problem. The flattened surfaces have acontour different from the adjacent surface, and are formed in thenucleus body. The term flattened section or flattened surface are usedinterchangeably and are meant to refer to any contour that is not thesame as the adjacent surface(s) of the nucleus. Such a flattened surfacecan be planar or it be slightly convex or concave and have a radius ofcurvature different from the adjacent surface. Such a flattened surfacecould also be in the shape of a compound curve or other complex shape.

This flattened surface can be angled relative to the superior end plateof the inferior vertebral body (or vice versa, or both), with the heightof the anterior end being greater than the height of the posterior endwhen lordotic correction is sought. The overall shape of the core canstill be asymmetric, but the flattened surface can be incorporated toprovide a reliable correction of the deformity. This flattened segmentprovides stabilization to resist the moments acting through the nucleus,i.e., if the flat is not of adequate size, there may be a tendency forthe correction to disappear in the presence of an anterior load or for ahyper-lordotic over correction in the presence of a posterior load(during lordotic correction). Another advantage of the flattened segmentis to provide surface contact over that area during small movementsabout the, neutral position of the device, which could help reduce wearon the device.

FIG. 1A illustrates a nucleus 10 that has not been adapted for lordoticcorrection because the upper and lower surfaces 22A and 22B are parallelto each other. In this nucleus, the axis 20 of greatest height falls inthe center of the disc. In FIG. 1B, a nucleus 14 that provides 3° ofcorrection is illustrated. This nucleus provides for lordoticcorrection. FIG. 1C illustrates another artificial disc nucleus 18having a greater degree of deformity correction. When deformitycorrection is provided as shown in FIGS. 1B and 1C, the geometric centerof the nucleus may shift to a location that is offset from the axis 20.

If the anterior/posterior directions are reversed, it provides akyphotic correction. If the nucleus is rotated 90 degrees, a scolioticcorrection is provided. In the illustration in FIG. 1C, the maximumvertical axis 20 is positioned to provide a correction of 6°. It isapparent that the nucleus can be adjusted to provide various degrees ofcorrection and, in certain cases, if no degree of correction is needed.Alternatively, only one of the halves of the nucleus 10 may have aflattened portion, with the other half having an outer surface that iscurved.

In FIGS. 2A through 2C, asymmetrical ovoid embodiments of an artificialnucleus are shown. The nucleus comprises upper and lower surfaces 22Aand 22B, which are “flattened” by virtue of the ovoid shape of thenucleus, upper and lower curved surfaces 24A and 24B, and acircumferential center portion 26. In the embodiments shown in FIGS. 2Band 2C, the maximum height axis 16 is asymmetrical with the geometriccenter 12 of the disc. In the nucleus shown in FIG. 2A, where there isno correction, the maximum vertical height is at the central verticalaxis 12. In the nucleus shown in FIG. 2B, the maximum vertical axis 16is positioned to provide an angle of correction of 3°. In the nucleusshown in FIG. 2C, the maximum vertical axis 16 is positioned to providean angle of correction of 6°.

FIG. 3 is a top view of one example of a nucleus. This nucleus 40comprises a central convex or flattened region 42, which includes agroove or slot 44. This groove or slot 44 enables the nucleus to slideonto the central keel or anchor of an end plate (not shown). While thenucleus 40 is shown as essentially circular, it is clearly apparent thatit may take on other shapes such as an ovoid or ellipsoid shape. It isalso clearly apparent that other types of anchor receiving means can beused. For example, the shape of the groove may vary or a snap-in orbayonet or dog-bone type of receptacle can be provided to anchor thenucleus in position. Those practiced in the art can provide additionallocking methods including the addition of one or more parts to the corethat provide an anchor.

For deformity correction, the nucleus may take the form of a truncatedcurved body as shown in FIG. 4. For this embodiment, the nucleus 50 hasan upper surface 52 that terminates in essentially flattened planar top54. A slot 56 or a groove or opening of another appropriate shape, canbe formed in upper surface 52 for receiving an anchor formed in the endplate. The lower surface 58 is typically an inverse of the uppersurface. However, instead of being truncated with a flat surface asshown in FIG. 4, the bottom surface could be asymmetrically spherical orovoid in shape.

Alternatively, the nucleus may be circular, ovoid or egg-shaped having anon-central maximum vertical axis as shown in FIG. 5. In anotherembodiment, the nucleus could be essentially circular or asymmetricallyspherical.

FIG. 5 illustrates an artificial nucleus 60 where the upper surface 62is an asymmetric convex surface. Again, either the top or the bottom orboth surfaces may be asymmetric.

For illustrative purposes, the nuclei in the figures have been shownadapted for lordotic correction. It is clearly apparent that the nucleuscan have an asymmetric maximum height at the front (anterior), the rear(posterior) or the side (lateral). The asymmetrical nucleus of thepresent invention can be used to correct for various types of spinalmisalignment including sagittal and coronal deformity,

The novel corrective nucleus of the present invention may be providedalone or it may be provided in combination with an upper end plate, alower end plate or both an upper and a lower end plate.

FIGS. 6 through 8 illustrate an exemplary artificial end plate 70 thatcan be used in conjunction with the nucleus to provide a novelartificial disc unit. An artificial end plate according to the presentinvention comprises an inner surface with a concave bearing surface forreceiving the convex surface of an artificial disc. The outer, or bonecontacting, surface is essentially flat.

To accommodate some previously known end plates, it was necessary tospend a significant amount of surgical time to prepare the vertebrae tothe appropriate shape to accommodate the artificial end plate. FIG. 6shows an end plate 70 with a flat outer surface 72 that enables the endplate to slide on the surface of the vertebra. One or moreunidirectional keels 76 are formed on the outer surface 72 to providefor immediate fixation. The keels may be placed centrally orparasagittaly. Fixation can be enhanced by incorporating onto the outersurface 72 a suitable coating 74, such as porous titanium, a calciumphosphate or the like, to promote bony ingrowth for long term stability.

A stop member 78 can be provided at the anterior edge 80 of the endplate. The stop member prevents the prosthesis from migratingposteriorly and possibly impinging on the spinal cord. An essentiallysemi-circular wall 82 joins the outer surface of the end plate to theinner surface. The thickness of 82 may vary with increased thicknessanteriorly, posteriorly or parasagittally, as discussed further below.The inner surface 84 is shown in greater detail in FIG. 7.

The inner surface 84 of the end plate articulates with the nucleus. Inthe embodiment shown in FIG. 7, this inner surface has a concave region86, which receives the nucleus. An anchor 88 is provided in the centerof the concave region 86 for positioning the nucleus and preventing itfrom migrating. The anchor 88 can be generally rectangular in shape withrounded edges, as shown, avoiding premature wear and cutting into thenucleus. FIG. 8 illustrates a front view of the end plate showing theouter surface 72 having two parasagittal keels 76 and the inner surface84 having a concave region 86 and a central anchor 88.

FIGS. 9-12 illustrate a nucleus and end plates described above assembledinto a TDR implant. FIGS. 9 and 10 show the use of a nucleus 96 with atruncated cylinder shape and a flattened portion 97 on the superior sideof the nucleus as described above, in conjunction with FIGS. 1A-1C, andFIGS. 11-12 show the same design with a nucleus 96 having an ovoid shapeas shown in FIGS. 2A-2C. In these figures, a complete spinal discprosthesis 90 comprising a superior end plate 92, an inferior end plate94 and an artificial disc nucleus 96 are provided. The end plates andnucleus can be provided in different sizes to accommodate differences inanatomy. The end plates and various nuclei can be provided in a kit tothe surgeon so that the appropriate sized components can be selected andused when the final size is determined. The end plates may be fabricatedof titanium or titanium alloy, chrome-cobalt-molybdenum (CoCrMo), cobalt28 chromium molybdenum, cobalt chrome, ceramics or other materialsuitable for spinal prosthetic implants.

The end plates have two distinct surfaces. The outer surface 98 is thesurface that contacts the vertebral end plate. The outer surface isessentially flat enabling it to easily contact the surface of thenatural vertebral end plate. The flat surface can be porous andincorporate a suitable treatment, such as porous titanium, a calciumphosphate or other types of known treatments such as coatings, plasmasprays, and structural changes to the surface, that promote bonyingrowth or ongrowth for long-term stability. At least one parasagittalkeel 100 is formed on the outer surface of each end plate to provideimmediate fixation.

As shown in FIGS. 9-12, three parasagittal keels 100 are aligned witheach other and located along both sides of the outer surface of the endplates. Alternatively, as shown FIG. 9A a similar end plate design withan upper end plate 92 and a lower end plate 94 have an offset keelconfiguration with a pair of aligned parasagittal keels 100A formed onthe outer surface of the upper end plate and a centrally-located row ofaligned keels 100B formed on the outer surface of the lower end plate94. This latter arrangement is believed to be advantageous because, withthe upper and lower keels being offset from each other, the end platesshould have greater stability and result in less stress on a vertebraewhere multiple implants are used.

Referring back to FIGS. 9-12, the inner surface 102 of each of the endplates has a concave region 103 or bearing surface that articulates withthe nucleus. An anchoring protrusion 104 projects outwardly from theconcave region, which provides an anchor for the nucleus and restrictsposterior translation. Both the superior and the inferior end plateshave flanges 106 for preventing the end plates from migrating into thespinal canal. The end plates can have holes 107 for allowing the endplates to be connected to the adjacent vertebrae through either metallicor bioabsorble screws (not shown) that can be inserted through holes107. FIGS. 9 and 11 illustrate front views of the prosthesis and FIGS.10 and 12 illustrate side views.

In another aspect of the invention, shown in FIGS. 13A-13B and 14A-14C,spinal deformity can be addressed by providing an artificial spinal discprosthesis where correction is provided in the end plates. Correctiveend plates may be provided alone, in combination with a symmetricalartificial nucleus that has flattened surfaces as described above onboth the top and bottom of the nucleus, as shown in FIGS. 13A-13B, or incombination with an asymmetrical nucleus that has flattened surfaces asdescribed above on both the top and bottom of the nucleus, as shown inFIGS. 14A-14C.

Correctional end plates are shown in FIGS. 13A-13B and 14A-14C. Thedegree of correction can be achieved by altering the inner(nucleus-contacting) side of the end plate or the outer(vertebral-contacting) side of the end plate. As shown in FIGS. 13A-13B,the end plate 110 comprises an outer (bone-contacting) surface 112, aninner surface 114, and a perimeter wall 116 connecting the outer andinner surfaces. The height of the perimeter wall 116 may vary accordingto the degree and type of correction required. For example, FIG. 13Billustrates an end plate adapted for a greater degree of correction thanthe end plate of FIG. 13A. The positioning of the variable height can beadjusted to treat different conditions such as lordosis, kyphosis orscoliosis. The inner surface may be shaped to receive the nucleus, andthe height of the end plate can be adjusted according to the degree ofcorrection required.

Alternatively, as shown in FIGS. 14A-14C, the outer surface 120 and theinner surface 122 may be essentially planar and the height is adjustedas the outer and inner surfaces become increasingly non-parallel as aresult of variation in the height of the perimeter wall 124. FIGS. 14Athrough 14C illustrate increasing degrees of correction, respectively.An advantage of having an essentially planar outer, orvertebral-contacting, surface is that the device is easier to insert andrequires less operating time to prepare the vertebral surface ascompared to traditional artificial disc devices.

FIGS. 15,16A and 16B illustrate another embodiment of the invention,which provides for all directions of movement, flexion/extension,lateral bending, and rotation about the symmetrical axis. In thisdesign, the nucleus 130 is formed in two sections 130A and 130B. A post132 is formed on the inner surface of one section 130A, and fits in anopening 134 that is formed on the inner surface of the other section130B to provide for relative rotational movement between the twosections 130A and 130B. The post 132 and opening 134 can be formed oneither section of the nucleus 130. The post and opening can be of anysuitable size, and can be perpendicular to the opposing surfaces of thenucleus sections 130A and 130B, or be tilted at an angle off horizontalto orient the axis of axial rotation with the anatomically correct axisand provide a deformity correction.

In this configuration, the contact surfaces between the nucleus 130 andend plates 136 and 138, are designed to have the same correspondingasymmetrical contours at the preferred angle between them, as shown inFIG. 15. Because there is only relative movement between the nucleus andthe end plates in the anterior/posterior and medial/lateral directions,greater surface contact between the nucleus and the respective endplates is possible in order to transmit rotations of the end plates tothe nucleus so that the two halves 130A and 130B, of the nucleus 130will rotate with respect to each other, rather than having the endplates 136 and 138, rotate on the outer surface of the nucleus 130.

FIGS. 17 and 18 show another embodiment of the invention where insteadof forming the nucleus 130 of a single piece of material, it can beformed of upper and lower sections 130A and 130B, with an intermediatesection 140, that is either flat or wedge-shaped as shown in FIG. 17,fixed to the upper and upper and lower sections. The intermediatesection 140 can provide the nucleus with the appropriate degree ofcorrection as shown in FIG. 18, instead of providing wedge-shaped endplates as discussed above. In a related embodiment of the invention, thenucleus 130 is essentially cut in half and has a flat inferior surface.This can be applied to the embodiment seen in FIGS. 17 and 18, where thesection 130B is removed, leaving the inferior surface of intermediatesection 140 articulating with the inferior end plate. By varying theconfiguration of the intermediate section 140, deformity correction canbe achieved.

FIGS. 19 and 20 show another embodiment of the invention where thenucleus 130 is essentially cut in half and has a flat lower inferiorsurface. This shape can be used to resist expulsion of a nucleus with anovoid/asymmetric shape, which could occur when the ovoid shape of thenucleus causes the end plates to tilt relative to each other to providecorrection. As shown, the bottom surface of the nucleus 130 is flat andis formed with a circular opening 134 that is shaped and positioned toreceive a post 136 formed on the opposing surface of the lower end plate138 for allowing relative rotational movement between the nucleus 130and the end plate 138. Alternatively, the nucleus could have the flatsurface and opening 134 on its upper or superior surface, instead ofbeing on the lower surface as shown. In this embodiment, the nucleus ispreferably asymmetrical as shown in FIG. 19.

A modification of the configuration in FIGS. 19 and 20, is shown inFIGS. 21 through 24, where the nucleus 130 is positioned in a slot ortrough 142 formed in the upper surface of the lower end plate 138. Asshown in FIG. 21, the undersurface of the upper end plate 136 iscontoured to match the nucleus. Alternatively, as shown in FIG. 24, theundersurface of the end plate 136 can be flat and engage a flattenedupper surface 146 of the nucleus 130.

In another embodiment, shown in FIG. 21A, lordotic correction isachieved in a prosthesis that replicates the movement of naturalvertebrae. A nucleus 130 is provided that has a concave configuration onits superior articulating surface 137, as discussed above. This concavearticulating surface 137 may incorporate a flattened segment as shownabove. The nucleus 130 is asymmetrical in that its superior surface 137is formed with an anterior aspect 145 a that has a greater verticaldimension, i.e., is thicker, than the posterior aspect 145 b. Thesuperior endplate 136 has an undersurface 147 that is similar in shapeand articulates relative to the superior surface 137 of the nucleus 130.The corresponding articulating surfaces 137 and 147 can be rounded inthe shape of a compound curve or other type of continuous curve, so thatthey are essentially congruent through a full range of motion Thesuperior surface of the bottom end plate 138 is formed with a definedrecess 139 that is highly polished, preferably to a mirror finish. Thenucleus 130 is positioned for movement in the recess 139, with theelevated edges that form the recess limiting movement of the nucleusrelative to the bottom end plate. The nucleus 130 can be sized andshaped to provide for controlled motion between the nucleus and thebottom end plate so that the nucleus 130 can slide in a gliding movementfrom side-to-side and/or from front-to-back relative to the superiorsurface 139 of the bottom end plate 138 to provide for relativetranslational movement in the medial-lateral and/or anterior-posteriordirections as indicated by the arrows 146 and 148.

The trough can be larger than the nucleus in both the anterior/posteriorand medial/lateral directions to allow for a desired amount oftranslation in those directions as shown by the arrows A and B in FIG.22. The trough can be open on the posterior or anterior end to allow thenucleus to be inserted simply by sliding it into the trough, as shown bythe arrow A in FIG. 21. In this way, the nucleus can be inserted withoutundue distraction of adjacent vertebrae. The nucleus can be preventedfrom moving out of the trough by providing a stop 144 of any suitablesize and shape. FIG. 23 is a schematic view of the nucleus 130 that isinserted in the trough in FIG. 21.

Another embodiment of the invention is shown in FIGS. 25-31, where thenucleus 130 is elongated, with a flattened section 150 that is a partialcylinder with curved sections 152 and 154 on both sides of the flattenedsection. It is believed that this design, when mated with a cylindricalsurface 156 on the interior of the upper end plate 136, shown in FIG.29, will provide better wear characteristics because it will havesurface contact during medial/lateral bending and line contact duringflexion/extension.

The elongated shape of the nucleus 130 is illustrated in FIGS. 25 and26, which show that the nucleus has a round cross section with constantmedial-lateral radius from anterior to posterior (A-P), with the flatsection 150 in the middle being oriented to provide a correction angleas described above, for the flatted portions on the other embodiments ofthe nucleus. The nucleus 130 is asymmetrical, with the flattened surface150 oriented at an angle and having a greater height at the anterior endthan at the posterior end. Thus when implanted between two vertebralbodies in the A-P orientation indicated in FIGS. 25 and 26, the nucleus130 has an asymmetrical shape aligned with the sagittal plane, andacross or crossing the coronal plane, of the vertebral bodies. Theinterior surface 156 of the upper end plate 136 has a cylindrical shapewith the same constant radius in the anterior/posterior direction as thenucleus.

In the neutral position, the cylindrical surface 156 mates with theflattened section 150 of the nucleus 130, and sits at an angle thatprovides a deformity correction as shown in FIG. 31. In this position,there is surface contact between the end plate 136 and the nucleus 130.During medial/lateral bending, there is also surface contact between theend plate and nucleus. During flexion/extension, with or without lateralbending, there is line contact between the end plate and nucleus. Thisconfiguration of core and end plate will always have line or surfacecontact, thus reducing the wear potential from point contact in some ofthe previous designs.

The elongated shape of the nucleus 130 allows for the end plate 138 tohave a trough 157 in the shape of a “bow tie” as shown in FIGS. 27 and28. This shape allows for axial rotation with stops beyond the limits ofnormal motion. The shape is oversized relative to the nucleus 130 by anappropriate amount to allow limited anterior/posterior andmedial/lateral translation. Additionally, the bottom surface of thetrough 157 can be rounded upwardly at the medial/lateral sides in FIG.30 (not shown), so that as the nucleus 130 rotates it is “cammed” upcausing a distraction of the device that forces the vertebral bodiesapart and loads the tissues between them resulting in a gradual stop tothe motion. Translation of the nucleus 130 within the trough 157 willtend to preserve the mobile instantaneous axis of rotation of thenatural disc.

FIGS. 32 and 33 show another embodiment of the invention, which utilizesa universal joint formed of a pair of cylinders 160 and 162 that rotaterelative to each about a central post 164 that projects from one of thecylinders 162 and engages an opening 166 in the other cylinder 160. Thecylinders 160 and 162 are oriented perpendicular to each other andengage cylindrical surfaces 168 and 170, respectively, in the adjacentend plates 136 and 138. This design provides for three anatomical axesof rotation. Because of the independence of each axis of rotation, anycorrection provided by the shape of the nucleus that is formed of thetwo cylinders will result in rotation to compensate for the correctionand a return to the uncorrected neutral position. Alternatively, thecylinders 160 and 162 may be shaped similarly to the elongated nucleus130 shown in FIGS. 25-27, or another suitable shape with a flat inferiorsurface.

Another embodiment of the invention is shown in FIGS. 34-36, where aresilient ring 172 and a post 174 that has a rounded top portion 176provide for relative motion between the end plates and for the desiredangle of correction. The ring 172 is shown in detail in FIG. 35. Thering 172 can be wedge shaped as shown in order to provide the desiredamount of correction, or it can be flat (not shown) if no correction isdesired. A projection 180 can be formed on the upper surface of thelower end plate 138 to mate with an opening 182 in the ring 172 in orderto prevent the ring 172 from moving relative to the lower end plate oncethe ring is in it desired position.

The upper end plate 136 has a cavity 178 that can be contoured to matchthe shape of the rounded top portion 176. The ring 172 is shaped so thatthe end plate 136 will ride on the ring 172 during “normal” ranges ofmotion, or through regular activities. However, when the normal rangesof motion are exceeded, then the ring 172 will compress and the upperend plate 136 will engage the post 174 causing the adjacent vertebrae todistract and thereby provide a gradual stopping motion or“anatomically-derived gradual stopping.” Alternatively, the post 174could be designed to serve as the primary load carrying part of thearticulation by riding in the cavity 178. In this design, the deformitycorrection force is only provided by compressing the ring 172. Thisdesign would have the advantage of reducing material stresses in theelastomer ring and creep.

As shown in FIG. 37, instead providing the ring 176, the same resultcould be achieved by providing two or more stops 184A and 184B, formedof a resilient material, between the two end plates. The stops 184A and184B can be mounted on the upper surface of the lower end plate 138. Oneof the stops 184A can project upwardly a greater distance than the otherstop 184B in order to provide the desired correction.

Another embodiment of the invention is shown in FIGS. 38 and 39, where anucleus 186 is provided that is formed of a resilient material that isshaped so that the nucleus provides medial/lateral rotation, butrequires deformation of the material during flexion and extension. Thisis accomplished by proving a central portion 188 that is spherical orovoid in shape and “flattened” adjacent end portions 190A and 190B thatare cylindrical, which extend the flattened end portions around thecircumference of the nucleus at both ends. The upper end plate has acavity (not shown) that has a contour that is similar in shape to thenucleus 186. A trough (not shown) similar to the one in FIGS. 27 and 28can be formed in the lower end plate 138.

For medial/lateral movement in the direction of the arrows A-A, theupper and lower end plates will rotate relative to each other throughrotational movement of the upper end plate on the nucleus 186. However,flexion/extension in the direction of arrows B-B will occur only throughdeformation of the nucleus 186. Alternatively, the nucleus 186 can berotated 90° on the lower end plate 138 so that so that the end plate 136will rotate on the nucleus during flexion/extension and the nucleus willdeform during medial/lateral movement. The end portion 190A has a largerdiameter than the end portion 190B to provide for the desired amount ofcorrection. As shown, the nucleus has been shaped so the resilience ofthe nucleus varies over its length. However, the nucleus could be formedof materials having varying degrees of resiliency along its length toachieve the same results.

FIGS. 40-43 illustrate another embodiment of the invention where the endplates 136 and 138 are provided with stops outside of the normal rangeof motion, which also utilize the concept of “anatomically-derivedgradual stopping” discussed above in conjunction with FIGS. 34 and 35.This type of stop can be added to any design that employs the use of endplates. This aspect of the invention is based on duplicating how thehuman body moves and then designing the cooperating surfaces to mimicthose motions as closely as possible. As shown in FIG. 40, the end plate136 has a post 200 on its lower surface that engages pocket 202 formedin the upper surface of the lower end plate and 138. Preferably, a pairof posts and pockets are provided on opposite sides of the nucleus 130.

As shown in FIGS. 40-43, the pocket 202 has a slot 204 in it with acurved surface 206 that is engaged by the lower end 208 of the post 200.As the end plates 136 and 138 move in the anterior/posterior directionrelative to each other during extension/flexion, the lower end 208 ofthe post rides along the curved surface 206. As the post reaches theouter limits of travel the lower end 208 will begin riding up thegradually curved section of the surface 206, which causes distractionbetween adjacent vertebrae as illustrated by the arrow A in FIG. 43 andloads the tissues between them, resulting in a gradual stop to themotion.

FIGS. 44 a, 44 b and 44 c illustrate another embodiment of the inventionwith a central depression or valley in the flattened segment on thesuperior surface of the core. In this embodiment the nucleus 130 hasfirst and second flattened sections 150 and 151 that are formed betweencurved sections 152 and 154. A shallow depression or valley 330 isformed in the flattened sections 150 and 151 and the curved section 152to provide an indention with slightly elevated margins 310, 320 that arerounded and contiguous with the adjacent surfaces 150, 151 and 152. Thedepression 330 is elongated in the anterior/posterior direction, alongthe sagittal plane SP. The depression 330 and margins 310 and 320 aresymmetric about the sagittal plane SP. A pair of transition zones 340and 350 extend between the depression 330 and the surrounding surfaces.This configuration is designed to potentially improve load distributionas the superior endplate moves relative to the nucleus because contactis distributed along the margins 310 and 320 instead being concentratedalong the mid-sagittal plane or other portion of the nucleus.

Another embodiment of the invention is shown in FIGS. 45, 45 a, 45 b and45 c, where the posterior superior surface 360 of the nucleus 130 isformed to curve upwardly toward the superior end plate 136 relative tothe superior surface 350 of the nucleus in order to provide a relativelyelevated posterior surface region 360. The superior end plate 136 has acorresponding surface 370 contoured to fit the posterior superiorsurface 360 of the nucleus. This configuration is designed to enhancethe stability of the prosthesis by allowing the endplates 136 and 138 toresist posterior shear when they are in a lordotic position. FIGS. 45 band 45 c illustrate the relative positions of the components in FIG. 45as the spine moves in flexion and extension, respectively. The elevatedsurface region 360 and its corresponding surface on the end plate 136causes shear loads to be transmitted through the nucleus and into theinferior endplate. Consequently, abnormal loads on facet joints of thenatural spine and related soft tissue structures are prevented.

FIG. 45 b illustrates another feature of the invention where the nucleus130 has a tang or tab 300 that extends posteriorly from theposterior-inferior aspect of the nucleus 130, which fits in an undercutportion or recess 302 of the bottom end plate 138. The tab 300 operatesto resist posterior migration (i.e., expulsion) of the nucleus towardthe spinal canal by preventing “lift off” of the nucleus from theinferior end plate and possible dislocation of the nucleus. As shown,the tab 300 can be rounded, chamfered or beveled in order to facilitateinitial insertion and intra-operative or post-operative replacement ofthe nucleus.

As shown in FIG. 44 a, a pair of shallow cavities or dimples 342 and344, are formed on opposite sides of the anterior aspect of the nucleusin order to allow a retrieval/insertion instrument to engage the nucleusfor insertion and removal. These cavities 342 and 344 are formed in alocation where they do not interfere with the load carrying capabilitiesof the nucleus.

FIGS. 46 a, 46 b, 46 c and 46 d show another embodiment of the inventionwhere a pair of keels 100 a are formed in the outer surface of the topend plate 92, with a bony ingrowth surface 101 a formed between thekeels 100 a. The lower surface of the bottom end plate 94 has a singlekeel 100 b formed along its anterior/posterior centerline, or sagittalplane, with a bony ingrowth surface 101 b formed on both sides of thekeel 100 b. The design of a pair of parasagittal keels on the superiorendplate and a single central keel on the inferior endplate, or viceversa, prevents “in line” saggital cuts in the vertebral body that canincrease the possibility of fracture of the body. Alternatively, a pairof keels could be formed on the inferior endplate and a single keel onthe superior could be used with the same beneficial results. This andother combinations of keels with an unequal number and/or locations ofkeels on the superior and inferior endplates will prevent excessivestress in the vertebral bodies when multiple spinal levels undergodevice implantation.

The keels 100 a and 100 b include a rail 400 attached to the respectiveend plates, with multiple teeth 402 in accordance with the size of thedevice, on each rail that projects in the anterior direction. The rails400 can include a number of holes 404 to allow for bony ingrowth foranchoring the rails in place after implantation. The bony ingrowthsurfaces can be any known type of surface that allows tissue to growinto or adhere to the upper end plate 92. Such a surface can includesintered beads, fibers, or other materials attached to the upper endplate, or the surface can be roughened or textured in a known way. Theingrowth surfaces 101 a and 101 b on the top and bottom plates areprotected by slight elevations or lips, which surround the bony ingrowthsurface (not shown). Tabs 406 can be provided to limit posteriormigration and for tooling locations for insertion instruments. A pair ofshallow cavities or dimples 408, 410, can be formed on opposite sides ofthe anterior aspect of the end plates 92, 94, respectively, in order toallow a retrieval/insertion instrument to engage the end plates forinsertion and removal.

In another aspect of the invention, all of the articulating surfaces ofthe prosthesis can be formed of a polymer. As discussed above, thenucleus can be formed entirely of a polymer such as, for example,ultra-high molecular weight polyethylene (“UHMWPE”), a cross, linkedUHMWPE, a ceramic, polyetheretherketone (“PEEK”) or other type ofsuitable polymer. The bony ingrowth surfaces can be made from plasmasprayed metals, hydroxyapatite or similar bone-like coatings, and caninclude a coating of bone growth factors. The articulating surfaces ofthe end plates can be formed with inserts of an appropriate polymer,ceramic or the like. The remaining exterior surfaces of the end platesthat interface with bone can be formed with bony ingrowth surfaces ofthe type discussed above.

FIGS. 47 and 48 illustrate a frame or scaffold 500 that is used todistract adjacent vertebral bodies 502 and 504 of the human spine,looking in the direction of the anterior surface of spine with thescaffold 500 in place. The scaffold 500 also operates as an instrumentguide for preparing adjacent bone surfaces on the vertebral bodies 502and 504. The scaffold 500 is held in place by locking mechanisms 490that attach to the vertebral bodies 502 and 504 through distracting pins492 that are connected to the vertebral bodies 502 and 504.

The vertebral bodies 502 and 504 are distracted in a known way by meansof a distracting instrument (not shown) that is connected between thedistracting pins 492 that are connected to the vertebral bodies fordistracting them or spreading them apart to create a space 509 betweenthem after the natural nucleus (not shown) is removed.

As the scaffold 500 is inserted between the vertebral bodies 502 and504, the locking mechanisms 490 are placed over the distracting pins 492and screws 491 are tightened to hold the scaffold 500 in place. As shownin FIG. 48, anchoring spikes 510 on the inner surface of the scaffold500 on opposite sides of the space 509 engage the vertebral bodies 502and 504 and provide additional support for anchoring the scaffold 500 tothe vertebral bodies.

A pair of lateral projections 506 on the inner surface of the scaffold500, extend posteriorly into the disc space 510, bilaterally, formaintaining the vertebral bodies 502 and 504 at a specific disc height.Grooves 514 are formed on the facing surfaces of the projections 506 forguiding instruments such as a trial, a drill guide and a keel shaperused during the implantation process, which are described in greaterdetail below.

FIG. 49 shows one of a set of trial instruments 516 with a handle 516 a,that can be inserted in the scaffold 500 or directly into the disc spaceto gauge AP/medial-lateral endplate coverage between the vertebralbodies 502 and 504 for determining the correct implant size that will beused. The side surfaces of the trial instruments 516 may have profiles515 that are shaped to fit into the grooves 514 formed on the scaffoldprojections 506. The trial instruments also may have two or more stops513 a and 513 b for preventing the instrument from being inserted toofar posteriorly between the endplates 502 and 504. The set of trialinstruments should be sized and shaped to at least have the capabilityof gauging various degrees of lordotic correction, preferably at least0°, 3° and 6° of lordotic correction, and to determine the size of theanterior-posterior and medial-lateral dimensions of the endplates forthe prosthesis.

FIG. 50 shows a drill guide 518 with a handle 518 a, and guide holes 520a, 520 b and 520 c formed in tabs 519 a, 519 b and 519 c, respectively,for setting the trajectory for drill bits (not shown) that are used tomake starter holes in the vertebral body 502 and 504, for grooves thatwill ultimately receive keels formed on the endplates of the prosthesisthat are described in greater detail above. The tabs 519 a, 519 b and519 c also operate as stops for preventing the drill guide from beinginserted too far posteriorly into the disc space 509 between thevertebral bodies 502 and 504. The drill guide 518 has side surfaces withprofiles 515 that are shaped to fit into the grooves 514 formed in thelateral projections 506 on the scaffold 500. The guide holes correspondto the location of the keels formed on outer surfaces of the upper andlower end plates of the prosthesis described above, and are used toguide drill bits (not shown) for forming starter holes (not shown) thatopen onto the facing surfaces of the vertebral bodies 502 and 504, sothat they can be shaped to receive the keels by the instrument shown inFIG. 52. FIG. 51 shows the scaffold 500 positioned between the vertebralbodies 502 and 504, ready to have the holes drilled in direction of thearrows.

Once the starter holes are drilled, a keel cutting instrument 522 of thetype shown in FIG. 52 is inserted in the scaffold 500 for shaping thestarter holes into grooves in the vertebral bodies 502 and 504. The keelcutting instrument 522 includes side surfaces with profiles 515 that areshaped to fit into the grooves 514 in the lateral projections 506 on thescaffold 500 so that the location and motion of the instrument can beprecisely controlled. The keel cutting instrument also includes sharpcutting edges 524 a, 524 b and 524 c that are used to shape the openingsthat were drilled into the vertebral bodies 502 and 504, with a simpleback-and-forth movement of the instrument relative to the scaffold 500.In this way, grooves of the exact shape are formed in the vertebralbodies for receiving the keels on the prosthesis with minimal forcebeing applied to the prosthesis. One or more stops 526 a, 526 b and 526c are provided on the instrument 515 for preventing the instrument frombeing inserted too far posteriorly between the vertebral bodies 502 and504 as the grooves are formed.

As shown in FIG. 52, described above, the keel cutting instrument 522has one keel cutter 524 a placed on one surface and two keel cutters 524b, 524 c on the opposite surface, although other configurations can usedas long as there is a shape such that at least one keel is on onesurface and at least two are on the other surface for providing maximumstabilization for the prosthesis and resisting torsional forces on theprosthesis. The trial instrument also has one or more stops 526 a, 526 band 526 c for preventing the instrument from being inserted too farbetween the endplates 502 and 504.

Alternatively, the drill guide 518 and keel cutter 522 can be replacedwith a drill guide 600 of the type shown in FIGS. 53A-53D. The drillguide 600 can include side surfaces with profiles 515 that are shaped tofit into the grooves 514 in the lateral projections 506 on the scaffold500 so that the location and motion of the instrument can be preciselycontrolled. In another technique, the scaffold 500 is not used and thedrill guide 600 is inserted into the disc space 509 with the aid ofintra-operative fluoroscopy. The drill guide 600 also can have a collar602 that includes three guide holes 604 a, 604 b and 604 c for guidingdrill bits 606 shown in FIGS. 53A and 53B. The collar also operates as astop to prevent the drill guide from moving to far posteriorly betweenthe vertebral bodies 502 and 504. The guide holes are oblong in shape sothat the drill bits can be moved in the guide holes to form not onlystarter holes, but also to form the grooves for the keels on theprosthesis. In this way, the keel cutter 522 described does not have tobe used as the groves for receiving the keels are formed solely throughthe use of the drill guide 600.

The final step of the process of implanting the prosthesis, after thegrooves for the keels are formed, includes assembling the prosthesis ofthe selected size and degree of lordotic correction, with the nucleussandwiched between the end plates. The physician uses an instrument thatis designed to engage the shallow cavities or dimples 408 and 410 on theend plates of the prosthesis described above. After lining up the keelsof the prosthesis with the grooves created in the vertebral bodies 502and 504, the prosthesis is then gently tapped into place between thevertebral bodies 502 and 504. The grooves formed as discussed aboveminimize the amount of force needed to position the prosthesis in theproper location between the vertebral bodies 502 and 504.

In the event that a core with an incorrect size or shape is inserted,the disc space could be distracted by various means, including vertebralbody distracting pins that are well known. An instrument with small pinsthat are designed to engage the small holes on the lateral aspect of thenucleus (as described previously) is used to remove the nucleus so thatit can be replaced with one having the correct size and shape.

Another embodiment of the invention that is designed to correct lordosisin the spine is a 2-piece device of the type shown in FIGS. 54, 55 and56, that allows normal kinematics while at the same time has awedge-shaped geometry. This design includes an elongated ball or“sausage shaped” articulating bearing surface, that is preferablylocated on the inferior endplate, that has its greatest diameter eitheranterior, posterior or lateral to the mid-portion of the inferiorendplate. This design would alter the articulation between the superiorand inferior endplates to provide a deformity correction.

As shown in FIGS. 54, 55 and 56, an elongated or “sausage type” shapedarticulating surface 500 can be incorporated into the inferior endplate502 such that the prosthesis has a 2-piece design, as opposed to the3-piece design described in previous embodiments. The elongatedarticulating surface 500 includes a flattened segment 504 and mates witha bearing surface 506 on the superior endplate 508. As shown in FIG. 54,the flattened segment 504 is incorporated onto the inferior bearingsurface to allow the resting configuration of the engaged endplates tobe in a lordotic configuration in the shape of a truncated cone. Withthis design, the inferior endplate 502 will have a maximum vertical axisthat is not located at the geometric center of the prosthesis, but islocated either toward the front of the endplate, the rear of theendplate or on one side of the endplate. The positioning of the maximumvertical height and load bearing capability is chosen depending on thetype of deformity that needs to be corrected. In FIG. 54, the maximumvertical axis is located toward the front of the endplate 502, whichwill provide lordotic correction.

The articulating bearing surface 500 of the inferior endplate 502 alsoincludes curved sections 510 and 512 on opposite sides of the flattenedsegment 504 to provide the full range of motion and translation in thesagittal and coronal planes.

The superior endplate 508 attaches to an upper vertebral member (notshown), and the inferior end plate 502 attaches to a lower vertebralmember (not shown). The endplates have an essentially flat outer orvertebral-contacting surface that allows them to be easily inserted.There can be keels or teeth 514, having the configuration discussed indetail above, to provide acute stability and anchoring of the device.The keels can be staggered to avoid stress concentration in the bone.There may be two parasagittal keels on the superior endplate and onemidline keel on the inferior endplate or vice versa. The outer surfaceof the end plates 502 and 508 may be treated in a way that promotes bonyingrowth to enhance stability of the end plate in situ. The end platecan include a stop member 516 to prevent the prosthesis from movingtoward the spinal canal.

The end plates can have two distinct surfaces. The flat surface of eachend plate, which contacts the vertebral body end plate, is capable ofaccommodating bony ingrowth and incorporates a suitable coating, such asporous titanium, a calcium phosphate, or includes other types of knownsurfaces mentioned above that promote bony ingrowth for long-termstability. The end plates can also have one or more parasagittal keelsthat provide immediate fixation.

The two pieces 502 and 508 may be composed of a low friction elastomersuch as polyurethane, polycarbonate-polyurethane, a polymer such aspolyethylene (particularly ultra-high molecular weight polyethylene), asuitable ceramic, metals or metal alloys such as titanium or a titaniumalloy, chrome-cobalt-molybdenum (CoCrMo), cobalt 28, chromiummolybdenum, cobalt chrome, stainless steel, PEEK or other suitablematerials. The flattened segment 504 of the inferior endplate 502 has agenerally cylindrical geometric design, with varying degrees of lordosisincorporated into it by utilizing an axis of maximum height anterior tothe geometric center of the inferior endplate. The anterior height ofthe inferior endplate varies, depending on the extent of lordoticcorrection needed. The inferior endplate can be made available invarious lordotic angles, e.g. 0, 3° and 6°, as well as differing heights(e.g., 4, 6 and 8 mm). Before deciding on the final device size, a setof instruments of the type described above or other means can be used togauge the need for lordotic correction.

As mentioned above, the articulating bearing surface 500 of the inferiorendplate 502 is elongated, with a flattened section that is either apartial cylinder or a partial cone, with curved sections 510 and 512 onboth sides of the flattened section. This shape is designed so that whenthe articulating bearing surface 500 of the inferior endplate is matedwith the cylindrical bearing surface 506 on the superior endplate 508,provide better wear characteristics will result because there will besurface contact during medial/lateral bending and line contact duringflexion/extension.

In the neutral position, the bearing surface 506 mates with theflattened bearing surface 504 and sits at an angle that provides thedesired deformity correction. In this position, there is surface contactbetween the superior end plate 502 and 508 and the inferior endplate.During medial/lateral bending, there will also be surface contact, butbetween the bearing surface 506 and one of the curved sections 510 and512.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. An intervertebral disc prosthesis comprising: a first end platesecurable to a first vertebral body of a portion of a spine, the firstend plate comprising a first articular surface; a second end platesecurable to a second vertebral body of the portion of the spine, thesecond end plate comprising a fourth articular surface; and a nucleuspositioned between the first and second end plates, the nucleus formedseparately from the first and second end plates, the nucleus comprisinga second articular surface that articulates with the first articularsurface and a third articular surface that articulates with the fourtharticular surface; wherein the nucleus comprises an asymmetrical shapeacross a coronal plane of the first and second vertebral bodies, theasymmetrical shape tending to urge the first and second end plates toremain in a relative orientation that provides a preferred lordotic orkyphotic angle between the first and second vertebral bodies; whereinline contact is provided between the first and second articular surfaceswhen the first articular surface articulates relative to the secondarticular surface in the anterior/posterior direction; and wherein thethird and fourth articular surfaces are substantially entirely planar topermit translational movement in at least one of the anterior-posteriorand medial-lateral directions.
 2. The intervertebral disc prosthesis ofclaim 1, wherein the nucleus comprises a maximum vertical axis, whereinthe maximum vertical axis is located anterior to the coronal plane ofthe first and second vertebral bodies to provide a lordotic correctionto the relative orientation between the first and second end plates. 3.The intervertebral disc prosthesis of claim 1, wherein the nucleuscomprises a maximum vertical axis, wherein the maximum vertical axis islocated posterior to the coronal plane of the first and second vertebralbodies to provide a kyphotic correction to the relative orientationbetween the first and second end plates.
 4. The intervertebral discprosthesis of claim 1, wherein at least one of the first and second boneengagement surfaces comprises at least one bone engagement featureshaped to penetrate bone.
 5. The intervertebral disc prosthesis of claim4, wherein the bone engagement features are distributed parasagittaly onthe bone engagement surface.
 6. The intervertebral disc prosthesis ofclaim 1, wherein the second articular surface comprises a first straightsection positioned between and contiguous with first and second curvedsections of the second articular surface, the first straight sectionextending across the coronal plane of the first and second vertebralbodies.
 7. The intervertebral disc prosthesis of claim 6, wherein thefirst articular surface comprises a first straight portion in at leastone cross section, wherein the first straight section rests against thefirst straight portion in a relative orientation between the first andsecond end plates that provides the preferred lordotic or kyphotic anglebetween the first and second vertebral bodies.
 8. The intervertebraldisc prosthesis of claim 6, wherein the third articular surface restsagainst the fourth articular surface in a relative orientation betweenthe first and second end plates that provides the preferred lordotic orkyphotic angle between the first and second vertebral bodies.
 9. Theintervertebral disc prosthesis of claim 6, wherein the first straightsection comprises a partial cylinder positioned between and contiguouswith first and second curved sections of the second articular surface,wherein the first articular surface comprises a cylindrical surfaceshaped to mate with the partial cylinder.
 10. The intervertebral discprosthesis of claim 6, wherein the line contact is between the firstarticular surface and at least one of the first and second curvedsections of the second articular surface.
 11. The intervertebral discprosthesis of claim 6, wherein the first straight section is oriented atan angle across the coronal plane of the first and second vertebralbodies.
 12. The intervertebral disc prosthesis of claim 1, wherein thenucleus and the second endplate are shaped to permit axial rotationbetween the third and fourth articular surfaces.
 13. The intervertebraldisc prosthesis of claim 12, wherein the second endplate comprises astop to limit the axial rotation between the third and fourth articularsurfaces.